Question

isJsonString('{ "Id": 1, "Name": "Coke" }')

should be true and

isJsonString('foo')
isJsonString('<div>foo</div>')

should be false.

I’m looking for a solution that doesn’t use try/catch because I have my debugger set to "break on all errors" and that causes it to break on invalid JSON strings.

Answer 1

A comment first. The question was about not using try/catch.
If you do not mind to use it, read the answer below.
Here we just check a JSON string using a regexp, and it will work in most cases, not all cases.

Have a look around the line 450 in https://github.com/douglascrockford/JSON-js/blob/master/json2.js

There is a regexp that check for a valid JSON, something like:

if (/^[\],:{}\s]*$/.test(text.replace(/\\["\\\/bfnrtu]/g, '@').
replace(/"[^"\\\n\r]*"|true|false|null|-?\d+(?:\.\d*)?(?:[eE][+\-]?\d+)?/g, ']').
replace(/(?:^|:|,)(?:\s*\[)+/g, ''))) {

  //the json is ok

}else{

  //the json is not ok

}

EDIT: The new version of json2.js makes a more advanced parsing than above, but still based on a regexp replace ( from the comment of @Mrchief )